How to Compare 3D CAD Models

11 Jun 2019 by Capvidia

cad-comparison

You may call it “3D CAD comparison,” or “CAD model validation.” We call it “comparative model validation.” But whatever you call it, comparing and validating your 3D CAD models is one of the most vital steps in the 3D CAD design process. And it’s a process you can’t do with your eyes. But more of that in a minute.

What is 3D CAD comparison?

3D CAD comparison is the process of comparing two CAD models to detect differences between them. The objective is validation. You are validating that the two models are identical in their geometry, 3D annotations, volume, surface area, center of mass. Your goal? Finding translation errors, detecting modeling errors introduced by humans and avoiding costly mistakes.

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Why should you do a 3D CAD comparison?

You want to perform comparative model validations on your 3D CAD models for one reason—to avoid costly blunders. Engineering is a precise science. When your tolerances are measured in thous (one thousandth of an inch) and microns (one thousandth of a millimeter) , you had better be sure that the part you are designing is the same as the part you are manufacturing. You do this by comparing two seemingly identical models, searching for differences.

The cost of overlooking variation in your 3D CAD models can be tremendous, especially in the age where CAD is so heavily utilized. For example, imagine a scenario where a machine tool is used to manufacture an expensive, high-precision aerospace component. Your manufacturing engineers will tell you that managing the machine tool errors, and staying in tolerance, is a very difficult and careful task. With modern machining, the NC toolpath is created from the 3D CAD model. What this means is that no matter how good your machine tool is, or how experienced your manufacturing engineers are, if you are starting with the wrong model, then you have introduced error into your manufacturing process before you even starting cutting metalThis is why comparative validation is so important—uncovering engineering errors prevents costly mistakes.

There are two types of 3D CAD Comparisons: Derivative Model Validation and Revision Comparison.

Derivative Model Validation

3D CAD designers often make copies of CAD models. They typically do this when they are working in a software platform that uses CAD models (CAM and CMM, for example) and the model they need is on another platform. For example, CAD software that designs a part is different from the machining software that creates the part. When a model is copied from one platform to another, translation is involved to define the model in the correct format.

In this process, the original model is called the authority (or the master), and the copy is called the derivative. Which means you now have two versions of the same model, but in different formats. As you can appreciate, it is vital that these two models are identical. The process of Derivative Model Validation compares the two models and looks for differences that may have been introduced during translation.

Comparing and validating 3D models ensures that the translation from one CAD system (CATIA for example) to another CAD/CAM/CMM system (Mastercam for example) has been done correctly.

If you don’t validate derivative models, you will introduce errors at the machining stage. Your CAM system or machine tool, for example, will create something that doesn’t look like or act like your part. The consequences can range from mildly embarrassing to catastrophic.

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Who needs to do Derivative Model Validation?

If you are a supplier to a large OEM, such as Boeing or General Motors, you should conduct Derivative Model Validation. The typical workflow is for an OEM to send their CAD model to suppliers to manufacture.

OEMs mostly use CATIA, NX, CREO and SolidWorks. Suppliers, on the other hand, use Mastercam, GibbsCam, SurfCam, CAMWorks and other CAD/CAE/CAM/CMM programs to create toolpaths for manufacturing the part. This means suppliers must translate the data they get from OEMs. And the rule is simple: “If you translate, you must validate.”

This is not necessarily an optional step, either. Boeing, for example, requires its suppliers to validate translations (Boeing D6-51991 Rev L). They have auditors that visit suppliers and request validation reports. The report proves that suppliers compared the Boeing model with the translated model and that both models were the same. Without this report, you don't win Boeing contracts.

If you translate, you must validate."

Revision Comparison

The second reason for doing 3D CAD comparison is to discover inadvertent changes between revisions. Revision Comparison looks at two versions of the same model and finds design changes.

Revision Comparison finds both intended and unintended design changes. Taking this step is vital for any well-controlled design process. Undocumented design changes can cost you in time, money and litigation. Many of our users have discovered that changing one part of their model causes unintended changes on another area of the model—without them knowing it at first. The consequences of such an undetected design change can be enormous. Revision comparison should be a part of your design and release process.

It’s also a useful tool for suppliers who need to easily understand changes between two versions of a model. For example, a supplier may prepare a quote before a design change is made. The supplier can easily find and review the design change to see if it affects the quote by increasing the cost of manufacture.

pmi-compare

How to Compare 3D CAD Models

There are two ways to compare and validate 3D CAD models—the wrong way and the right way. The wrong way is to eyeball it. You simply bring both models into your CAD system, overlay one on the other and start looking for differences. This is dumb for three reasons.

Reason 1: Your eyes can’t detect differences down to the micron level. Even someone with 20/20 vision can’t see one thou (one thousandth of an inch) of a difference between one part and another. Human eyes cannot detect the deviations that cause costly manufacturing errors.

Reason 2: Computer monitors can’t display differences between parts down to one thousandth of an inch. The pixel on your monitor is larger than the error you are hunting for.

Reason 3: 3D CAD models, by definition, are complex. Humans can’t detect the differences between two models.

The right way to compare and validate 3D CAD models is with dedicated software. A product like CompareVidia, for example, detects and identifies even the smallest errors or changes in geometry, topology and annotation (including PMI), letting you reuse your CAD data with confidence.

3D CAD model comparison software validates CAD translations made by suppliers, identifies and helps manage shape changes, documents the validation process, and satisfies the requirements of the strictest auditors (the Boeings of the world, for example).

Conclusion

Comparing and validating your 3D CAD models is a crucial step in contemporary manufacturing. If satisfied customers, safety and regulatory compliance are important to you, start using a specialized application to compare and validate your 3D CAD models.

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